Sound system



March 27, 1934- H. J. HASBRCUCK Er AL SOUND SYS TEM Filed June 27, 1931 :I: 2...... ...12.1. .....vvvvv Patented Mar. 27, 1934 UNITED srarss PATENT GFFHCE -wognum SOUND SYSTEM Application June 27, 193i, Serial No. 547,332

6 Claims.

This invention relates to methods and means for damping diaphragms such as are employed in devices for translating, recording or reproducing sound and more particularly to the control of low frequency vibrations, including resonant vibrations, of such diaphragnis which have little elastic restoring force.

An object of the present invention is to control the low frequency response of a diaphragm 10 of the type having a natural period of vibration in the lower audio frequency range.

Another object is to damp the resonant vibration of an acoustic diaphragm of the aforementioned type.

A further object is to adjust the degree or amount of such response or damping to a desired Value.

Oneror more of these objects are accomplished by substantially enclosing a space adjacent one side of the diaphragm in which the movement of air resulting from the movement of the diaphragm is restricted.

The effect of this restriction on the vibration or response of the diaphragm depends upon the degree of confinement of the air adjacent the diaphragm, the size of the space and the extent to which the diaphragm tends to move, the effect being greater for larger amplitudes. For uniform response the diaphragm must vibrate at greater amplitudes for low frequencies than for high frequencies, hence the damping effect of the air in the confined space is greater for vibrations in the lower range of sound frequencies.

For practical considerations the damping effect should not depend entirely on the physical dimensions of the parts or the clearance between parts of the device. It is desirable, in devices such as microphones of the type herein considered, that the damping be adjustable after assembly. This is accomplished by providing air passages from the restricted space near the diaphragm to points removed from the diaphragm thereby relieving the restriction, preferably to such an extent that resonance of the diaphragm is noticeable, and then restricting the passages sufiiciently to obtain the desired air restriction and consequent damping of the diaphragm.

The present invention is particularly useful for suppressing the natural resonant vibrations o of diaphragrns having little elastic restoring force, such as are generally employed in electrodynamic or moving coil translating devices. It has been proposed to malte the resonant frequency so low that it is outside the useful range of audio frequencies but with improved auxiliary apparatus this range has been extended and now includes very low frequencies, thereby imposing severe limitations on the design of a vibratile element if the resonant frequency thereof is to be kept outside the useful range. The present invention permits the resonant frequency of a diaphragm to be Within the useful range and provides means and methods for reducing excessive vibration of the diaphragm at resonance by selected amounts to render the response characten istie of the diaphragm substantially uniform.

Other objocts and advantages will be apparent from the following description of one embodiment of this invention when read in connection with the figures of the accompanying drawing 70 wherein:

Fig. l represents a section through the centre of a translating device employing vibration control in accordance with this invention;

Fig. 2 is a plan View of the vibration control member or iiller of the device shown in Fig. l;

Fig. 3 is a sectional view of a modification of the ller shown in Fig. 2; and

Fig. e is a view of an alternative form of the device of this invention.

Referring to r'ig. l, reference character l represents a magnetic structure having an air gap 2 and a coil of insulated wire 3 for creating a magnetic flux in structure 'l and gap 2 when energized from a source of electric current applied to connecting leads 4. The magnetic structure 1 preferably comprises a cylinder of magnetic material which has been hollowed out to provide an annular space for coil 3 between a central core 6 and outer Wall '7, and a cover plate 8 having a central hole 9 forming one magnetic pole for air gap 2. The other poles for this gap is formed by an end portion of core 6. The two parts of the magnetic structure may be held together in any suitable manner such as by screws lo recessed in plate 8 threaded into Wall 7. A diaphragm 11, preferably conical in shape, carries a coil of insulated wire l2 disposed in gap 2, suitable leads 1 being provided therefor. Diaphragm ll is 0 ported by means which permit movement l@ cloth or kid skin may be clamped between rings 19 and 20.

A vibration control member 22, which is the principal feature of this invention, is inserted between diaphragm 11 and plate 8 and fills substantially all of the space therebetween except a portion 2l closely adjacent the diaphragm. Filler 22 is preferably made solid and of metal such as aluminum. In addition to controlling vibrations of the diaphragm, member 22 may function as a support or spacer for clamping rings 19 and 20 and suspension 15. All of these parts may be secured to plate 8 by screws 23, thus providing means for supporting the member 22 and diaphragm 11 in proper relation to each other and to the magnetic structure.

Fig. 2 shows the vibration damper or ller 22 in greater detail. A plurality of holes 24 are provided near its periphery for the screws such as 23, and a central hole 25 is made equal to or somewhat larger than hole 9 in plate 8, so that coil 12 may pass through it. To obtain the desired damping effect a plurality of holes or passages 26 are provided from an outer surface or the periphery of filler 22, to another surface 27 thereof. When the parts are assembled as shown in Fig. l, surface 27 is disposed close to but not touching diaphragm 11 and defines the air space 21 between member 22 and the diaphragm, Suitable means for stopping up one or more of the holes 25 are provided such as cord Stoppers 28.

Fig. 3 represents a modication in which a filler 30, similar to ller 22, is provided with a plurality of threaded holes such as 31. Threaded plugs such as 32 may then be screwed into holes such as 31. This arrangement permits each plug to be positioned at any point in the length of the corresponding hole.

In operation the device shown in Fig. 1 may be used as either a loud speaker or microphone, depending on whether coil 12 is energized from a source of periodic current representing sound or sound waves are caused to impinge on diaphragrn 11 thereby generating a periodic current in the coil. In either case coil 3 is energized from a source of substantially continuous current in the usual manner to supply a strong magnetic flux in the gap 2. The natural period of vibration of the moving element comprising cone 11, coil 12 and portions of their suspensions le and 15, is conveniently located somewhere between 'l0 `and 125 cycles but other constructions may resuit in diierent fundamental frequencies. In any case, however, the response of diaphragm 11 at its resonant frequency may be out of proportion to the energizing force either in the form of air waves striking the diaphragm or periodic current in coil 12. Thisl abnormal response of diaphragm 11 at resonance is reduced as desired by the restricted air space provided by filler 22. It is usually desirable to reduce the resonant response to a point where this response is proportional to that of other frequencies, that is, to obtain substantially uniform response for all frequencies within the range of the device. This result may be obtained solely by the use of, member 22, if its dimensions are correct, or if not correct the damping effect may be varied by changing the dimensions to vary the width of air space 21 or the area of this space, that is, the area of the surface of member 22 which is opposite the surface of cone l1. This arrangement for damping control is not convenient, however, because it requires changes in physical dimensions of the parts or their relative positions. The filler 22 and air space 21 are preferably dimensioned to cause over-damping of diaphragm 11 if there are no holes such as 25 in the filler. Such holes are provided, however, in sufficient number to relieve the restriction of air movement in space 21 to such an extent that diaphragm 1l is underdamped. Stoppers 28 or plugs 32 are then inserted in a suiiicient number of holes 25, or the holes stopped by other means, to obtain the desired degree of damping which, in practice, is that degree of damping which will give a substantially flat response curve.

The previously described embodiment is the preferred form, but the embodiment shown in Fig. 4 also produces desirable results. In this embodiment a held structure, having a coil, a core, and a case as shown in Fig. l is utilized, with a front pole plate 8 as indicated. A similar cone member 11, is utilized supported from plate members 19 and 20 as in the iirst embodiment. The rings 19 and 20 are supported by spacer members 35 as shown, and held in place by simllar screws 23. A band member 3G, adapted to surround the rings 19 and 2i), over the supports 35 is provided. equipped with clamping screws 3i', and a felt liner member 38. This member 36 is placed over the rings 19 and 20 and the spacer members 35 and may then be adjusted axially on the spacer members 35 to bring the edge thereof more or less close to the member 8, thereby providing an adjustable openF ing for air vibrations from the conned space dened by it behind the diaphragm 11.

This embodiment provides a larger volumetric space behind the diaphragm l1, than the previously described embodiment provides. For some purposes, however, this larger space is preferable. It is also conveniently adjustable to control the outlet area, and thereby the damping.

The embodiments of the invention illustrated and described herein have been selected for the purpose of clearly setting forth the principles involved. The invention is, however, susceptible of being further modified to meet different conditions encountered in its use without departure from the inventive concept disclosed and it is therefore intended that the appended claims cover all modifications within the spirit and scope of the invention.

We claim:

1. An acoustic device comprising a vibratile conical diaphragm and a iixed member having a conical surface disposed adjacent to but outside of and not touching said diaphragm, said member being provided with a plurality of passages extending from the conical surface to another surface thereof to control the vibration of said diaphragm.

2. An electrodynamic microphone comprising a magnetic structure having an annular air gap therein, a diaphragm bearing a coil disposed in the gap, a member which lls substantially all of the space surrounding the gap between one end of said magnetic structure and said diaphragm except an annular portion thereof dened by opposed surfaces of the diaphragm and member, the said member being provided with a plurality of holes communicating with said annular space and extending to an outer surface of said member, and means for stopping one or more of said holes.

3. An electrodynamic microphone comprising a magnetic structure having an air gap therein, a frustum of a cone bearing a coil disposed in the air gap, means for flexibly suspending said cone loo for vibratory motion in the direction of its axis, and a ring of solid material substantially filling the space between said cone and said magnetic structure and having a conical surface substantially equal in area to one surface of said cone disposed parallel and adjacent thereto, whereby the said parallel surfaces dene a layer of air for damping the vibratory motion of said cone, said ring of solid material being provided with a plurality of passages opening at one end thereof into the said layer of air and at the other end thereof into free air at points removed from said cone.

4. A microphone comprising a vibratile conical diaphragm, a fixed member having a conical surface disposed adjacent to but outside of and not touching said diaphragm, said member being provided with a plurality of passages leading from the conical surface to the open air to control the vibration of said diaphragm, means for supporting said diaphragm, and means for driving said diaphragm.

5. A microphone comprising a vibratile diaphragm, a fixed member supporting said diaphragm having a surface disposed adjacent to but not touching said diaphragm, a coil mounted on said diaphragm, said member being provided with a plurality of passages extending laterally from said surface to the open air, the length of said passages being greater than the Width thereof.

6. A microphone comprising a magnet, a back plate supported thereby, a diaphragm, means for mounting said diaphragm in close face to face relation with said back plate, a longitudinally movable coil associated with said magnet and mounted on said diaphragm, and laterally extending passages in said back plate communicating With the inner face of said diaphragm and with the open air.

HAROLD J. HASBROUCK. ROBERT H. DREISBACH. 

